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Definition

Functional Residual Capacity is the volume of air present in the lungs, specifically the parenchyma tissues, at the end of passive expiration. At FRC, the elastic recoil forces of the lungs and chest wall are equal but opposite and there is no exertion by the diaphragm or other respiratory muscles.
FRC is the sum of Expiratory Reserve Volume and Residual Volume and measures approximately 2400 ml in a 70 kg, average-sized male. It can not be estimated through spirometry, since it includes the residual volume. In order to measure RV precisely, one would need to perform a test such as nitrogen washout, helium dilution or body plethysmography.
A lowered or elevated FRC is often an indication of some form of respiratory disease. For instance, in emphysema, the lungs are more compliant and therefore are more susceptible to the outward recoil forces of the chest wall. Emphysema patients often have noticeably broader chests because they are breathing at larger volumes. In healthy humans, FRC changes with body posture.
The helium dilution technique and pulmonary plethysmograph are two common ways of measuring the functional residual capacity of the lungs.